As a power source of a mobile device, a lithium ion secondary battery is mainly used for the reason of its high-energy density and long cycle life. The function of the mobile device or the like is diversified, resulting in increasing in power consumption thereof. Therefore, a lithium ion secondary battery is required to have an increased energy capacity and, simultaneously, to have an enhanced charge/discharge cycle characteristic. Further, there is an increasing demand recently for a secondary battery with a high output and a large capacity for electric tools such as an electric drill and a hybrid automobile. In this field, conventionally, a lead secondary battery, a nickel-cadmium secondary battery, and a nickel-hydrogen secondary battery are mainly used. A small and light lithium ion secondary battery with high energy density is highly expected, and there is a demand for a lithium ion secondary battery excellent in large current load characteristics.
In particular, in applications for automobiles, such as battery electric vehicles (BEV) and hybrid electric vehicles (HEV), a long-term cycle characteristic over 10 years and a large current load characteristic for driving a high-power motor are mainly required, and a high volume energy density is also required for extending a driving distance, which are severe as compared to mobile applications.
In the lithium ion secondary battery, generally, metal oxide such as lithium cobaltate and lithium manganese; and composite oxide thereof are used as a positive electrode active material, a lithium salt is used as an electrolyte, and a carbonaceous material such as graphite is used as a negative electrode active material.
Graphite used for a negative electrode active material is classified into natural graphite and artificial graphite.
Generally, natural graphite has advantages of low cost and high capacity due to its high crystallinity. However, as natural graphite has a scale shape, if natural graphite is formed into a paste together with a binder and applied to a collector, natural graphite is aligned in one direction. When an electrode made of such a material is charged, the electrode expands only in one direction, which degrades the performance of the electrode such as current characteristics and cycle life. Spherical natural graphite, which has been made by granulating natural graphite to be formed into a spherical shape, is proposed, however, the resulting spherical natural graphite is aligned because of being pulverized by pressing in the course of electrode production. Further, as a demerit of high crystallinity, the surface of the natural graphite is active, and hence a large amount of gas is generated during initial charging, which decreases an initial efficiency and further degrades a cycle life. In order to solve those problems, Japanese Patent publication No. 3534391 (U.S. Pat. No. 6,632,569, Patent Document 1), etc. propose a method involving coating artificial carbon on the surface of the natural graphite processed into a spherical shape. The material produced by this method can address a high-capacity, a low-current, and an intermediate-cycle characteristic required by the mobile applications, etc. However, it is very difficult for the material to satisfy the requests such as a large current and an ultralong-term cycle characteristic of a large battery as described above. In addition, natural graphite has a problem relating to quality stability because it contains a large amount of metallic impurities such as iron.
On the other hand, regarding artificial graphite, first, there is exemplified a mesocarbon microsphere-graphitized article described in JP 04-190555 A (Patent Document 2). This is a well-balanced negative electrode material, and is capable of producing a battery with a high capacity and a large current. However, it is difficult to achieve the cycle characteristics for a long period of time which are required for a large battery due to problems such that the conductive contact of the electrode powder with each other tends to degrade.
As the artificial graphite, articles obtained by graphitizing petroleum, coal pitch, coke, and the like are available at a relatively low cost. However, a satisfactory crystalline needle coke tends to align in a scale shape. In order to solve this problem, the method described in JP 3361510 B2 (EP 0918040 B; Patent Document 3), etc. yield results. This method can allow the use of not only fine powder of an artificial graphite raw material but also fine powder of natural graphite, or the like, and provides high capacity and excellent properties for a conventional graphite for a small lithium ion secondary battery. However, an improvement in productivity, a reduction in production cost, impurity management, improvements in cycle characteristics and storage characteristics, and the like toward an increase in use amount are indispensable to the satisfaction of characteristics required in automobile applications.
For example, the following methods have each been known as a method of graphitizing a carbon raw material powder as an ungraphitized product in a production process for an artificial graphite-based material to be used in the negative electrode of a lithium ion secondary battery:
(1) the carbon raw material powder is filled into a crucible made of graphite and graphitized with an Acheson furnace (JP 3838618 B2 (U.S. Pat. No. 6,783,747 A); Patent Document 4);
(2) the carbon raw material powder is molded into a certain shape with a binder such as a pitch or a polymer and graphitized with an Acheson furnace, and then a molding is shredded (Patent Document 3);
(3) the carbon raw material powder is loaded into a container made of a graphite material and heated with a heater as a heat source to be graphitized; and
(4) the carbon raw material powder or a molding thereof is moved in a space heated with a heater.